Method of inhibiting undesirable radio-



1964 KANG YANG ETAL 3,143,509

METHOD OF INHIBITING UNDESIRABLE RADIOACTIVE LABELING AND RADIATIONDAMAGE IN SYSTEMS CONTAINING MATERIAL UNDERGOING RADIOACTIVE DECAY FiledMarch 15, 1961 O O O m TIME (HRS) KANG YANG PRESTON L. GANT INVENTORS.

2; TORNEY United States Patent 3,143,509 METHOD OF INHIBITINGUNDESIRABLE RADHO- ACTIVE LABELING AND RADIATION DAMAGE IN SYSTEMSCONTAINING MATERIAL UNDER- GOING RADIOACTIVE DECAY Kang Yang and PrestonL. Gant, Ponca City, Okla, as-

signors to Continental Oil Company, Ponca City, Okla, a corporation ofDelaware Filed Mar. 15, 1961, Ser. No. 96,018 7 Claims. (Cl. 252-3011)This invention relates to a method for inhibiting undesirableradioactive labeling and radiation damage in systems containing at leastone component which emits high energy electrons or gamma photons inundergoing radioactive decay. More particularly, but not by way oflimitation, the present invention relates to a method of suppressing thedegradation and alteration of molecules resulting from subjecting suchmolecules to impingement by high energy electrons produced duringradioactive decay of radioactive materials. In a more specific aspect,the present invention relates to a method for storing tritium andtritium-labeled compounds so that the formation of undesirable compoundsin the system is inhibited.

Compounds labeled with radioactive elements play important roles in theinvestigation of numerous problems in chemistry and biology. Thesynthesis of such compounds is frequently quite laborious andtime-consuming. After the compounds have been prepared, it is oftendesired to store them for subsequent experiments. In the past, thestorage of such labeled compounds has been very difficult due to thefact that the radioactive tracer atom underwent uncontrollableradioactive disintegration, and the resulting radiation not only damagedthe labeled compounds but also produced various undesirable products.

A closely related problem is encountered in the storage and preservationof certain radioactive isotopes. For example, when tritium is stored inglass or steel containers, small amounts of methane contaminate thesystem over short periods of time, and the radioactive decay of thetritium leads to radioactive labeling of the methane as manifested inthe formation of tritiated methane. Even the purest tritium which iscommercially available contains tritiated methane in quantitiessufficient to give various undesirable side products when the tritium isreacted with organic compounds for the purpose of producingtritium-labeled compounds The tritiated methane which is present alsointerferes with mechanism studies of radiolysis reactions initiated withthe B decay of tritium. When the impurity is removed from the tritium bylaborious and expensive purification techniques, the tritium againbecomes contaminated in a relatively short period of time, apparentlydue to small amounts of organic material in the storage container andnot readily removable therefrom.

Whether the system stored be one containing only radioactively-labeledcompounds, or radioactively-labeled compounds in the presence of adiluent, or simply radioactive isotopes having traces of organicmaterial present, the formation of undesirable products in the systemand the accelerated decomposition of the labeled compounds originates inthe uncontrollable radioactive decay or disintegration of theradioactive atoms in the system. We have studied the reaction mechanismof this radioactive disintegration and the manner in which the decayproducts influence the formation of undesirable compounds or acceleratethe decomposition of the labeled compounds initially present in thesystem. These studies have led to the discovery that the formation offree radicals is an important step in the process of such formation andaccelerated decomposition, and the present invention is bottomed uponthis discovery.

3,1435% Patented Aug. 4, 1964 It is known that undesirable free radicalreactions may be inhibited by the addition of certain radical-scavengingcompounds to a system containing free radical-producing materials andmaterials which are attacked by, or will react with, such free radicals.The present invention can therefore be said to reside in the discoverythat free radicals are produced in certain systems containingradioactive elements and that the undesirable effects of subsequentreactions of these radicals with compounds or other radicals present inthe system can be inhibited by the introduction of certain compounds tothe system which will react with the radicals to form stable compoundsnot readily undergoing subsequent decomposition or further reaction. Theselection of the type of radical-scavenger employed is dependent uponthe compounds of the system which are to be protected, while the amountor quantity of scavenger to be added is dependent upon the period ofstorage.

Accordingly, it is a major object of the present invention to provide amethod for inhibiting radiation damage in a system containing at leastone compound which undergoes spontaneous radioactive decay.

It is a further object of the present invention to provide a method forinhibiting the formation of undesirable products resulting fromradioactive decay in a system containing a radioactive isotope and smallquantities of organic material.

It is another object of the present invention to provide a method forinhibiting radiation damage to chemical compounds labeled withradioactive tracer atoms and to unlabeled organic compounds used asdiluents in storing such labeled compounds. The invention also has asits object the inhibition of the formation of undesirable products insystems which include such labeled compounds and diluents.

The manner in which the cited objects are achieved will be betterunderstood upon reading the following disclosure, whereupon additionalobjects not hereinbefore described will become manifest.

The sole accompanying drawing is a typical curve in which the tritiatedmethane/ tritium ratio of the system is plotted against time andillustrates the manner in which purified tritium gas is rapidlycontaminated with tritiated methane when purified tritium is merelyallowed to stand in a Pyrex storage bulb.

In early studies of radioactive materials, it was observed that ,B rays,soon recognized as very energetic electrons, were emitted spontaneouslyby a large number of radioactive materials. The positively charged ionswhich remain after emission of B rays then undergo furtherdecomposition. The high energy 5 rays, on the other hand, are absorbedby the materials of the system containing the radioactive component, andtheir energy may be expended in the formation of additional ions and/ oratoms and molecules of activated or high energy status, since the energyrequired to alter the extra-nuclear electron arrangement of a moleculeis less than the energy of the emitted B rays.

From this knowledge of the ,8 decay process, an observation of the typesof compounds produced in gaseous systems as the end products of thedecay of certain radioactive materials, and an observation of thereaction conditions obtaining in such systems, particularly thetemperature coefiicients involved in the formation of certain products,we postulated that free radicals are formed as intermediates in thereactions induced in the system by the absorption of the {3 rays. Thiswas confirmed by the addition to the system of compounds which werecapable of reacting with the free radicals so formed to produce stablecompounds not readily undergoing further reaction. Since the addition ofsuch radical-scavenging compounds resulted in a substantial decrease inthe rate of decomposition of the components of the system and in therate of formation of decomposition products, the transitory existence ofsuch free radicals was confirmed.

The efiectiveness of the suppression of radiation damage and undesirabledecomposition in gaseous systems as a result of the addition of suitableradical-scavenging compounds is demonstrated by the following examples:

EXAMPLE 1 Tritiated Propane-Propane System The mechanism of thedecomposition of tritiated propane gas while standing in a storage bulbcan be divided into two categories:

Category (A): Direct decomposition due to the radioactive decay of thetritium atom with the emission of ,8 rays. The Category (A)decomposition process can be represented by mechanism reactions:

CH CH CH (He Decomposition and formation of products Category (B):Indirect decomposition due to ionizing radiation with [3 rays emitted inthe direct decomposition of Category (A).

The Category (B) decomposition process can be represented by themechanism equation:

e*(,8 rays) |CH CH CH T Formation of C H T+ ions, hot tritiated propanemolecules and ultimate decomposition When the tritiated propane isstored in the presence of propane as a diluent, an analogous equationmay be used to represent the impingement of the 3 rays upon the propanemolecule with resulting decomposition, thusly:

e (fl rays) +CH CH CH Formation of C H ions, hot propane molecules andultimate decomposition Obviously there is no way of preventing thespontaneous, direct decomposition of Category (A) from occurring.However, direct radioactive decay contributes negligibly to the over-alltotal decomposition in the system, the major role in the decompositionbeing played by the indirect decomposition resulting from molecularabsorption of the high energy {3 rays (Category B). This becomesapparent when one considers that the average energy of electrons emittedin the B decay of tritium is 5.57 10 electron volts, and that itrequires'only 24 electron volts of energy to produce a C H ion and anelectron from the propane molecule. Since under most storage conditionsnearly all of the B radiation energy is absorbed by the propane andtritiated propane molecules of the system, there therefore results morethan two hundred C H and C H T+ ions which subsequently undergodecomposition or enter into the formation of new compounds. Thus, theCategory (A) decomposition contributes, at most, 0.5 percent of thetotal decomposition.

The C H and C H T+ ions formed by the Category (B) mechanism thendissociate into free radicals. The free radicals so formed, as is wellknown, possess great chemical reactivity and many have such high energycontent that they react upon collision with normal molecules, therebyforming a new and stable molecule of different structure and a new anddifierent free radical, which again may react with a normal molecule andso on until eventually two active radicals collide and combine. Themechanism of decomposition and side product formation which is initiatedin a propane=tritiated propane system by e radiation (Category B) maythus be represented by the equations:

a s e- H, CH3, 02m etc. H, CH3, 0,115 cans, C3H1T Products (FreeRadicals of hydrogen and normally gaseous paraflins.)

We have found that the decomposition of the components of thepropane-tritiated propane system may be inhibited, and the formation ofside products suppressed by the introduction of a small amount ofbutadiene (normally gaseous diolefin) to the gaseous. system. Thiscompound reacts readily with methyl and ethyl free radicals to give highmolecular weight, nongaseous products which can readily be removed fromthe storage bulb. It also reacts readily with the thermalized hydrogenatoms produced in the system. The amount of the scavenger which is to beadded will, of course, depend upon the period of storage.

EXAMPLE 2 T ritium-Methane System The mechanism of the radioactive decayof tritium may be expressed by the equation:

z- "(fi y The [3 rays are then absorbed by the tritium and methane ofthe system and the probable mechanism of the resulting reactions is:

The thermalized and hot, or energetic, free radicals so formed thenenter into sundry reactions by which several undesirable side productsare formed, the most prevalent of which is tritiated methane.

To further establish the role of the 18 rays (as opposed .to thepositive ions formed upon decay of the tritium) in the formation of thetritiated methane, a 156 cc. storage bulb containing purified tritiumwith traces of methane was subjected to gamma photon irradiation. Thephoton source was four spent fuel elements from the Materials TestingReactor, Arco, Idaho. The gaseous mixture was irradiated for eighteenhours at 4 10 roentgens/hour. A sharp increase in the initial rate ofcontamination of the system with tritiated methane was observed. Thus,

it is estimated that practically all of the contamination resulted frommolecular absorption of B rays rather than from formation of products byreaction of the positive ion (He T)+ formed in the radioactive decay ofthe tritium.

To determine the effect of the addition of a suitable radical-scavengingcompound to the system, 10 mm. of

mercury of nitric oxide gas was introduced into a 156 cc. storage bulband then tritium gas containing a small amount of methane was introduced'into the bulb. Under this condition, tritiated methane was notdetectible in the bulb even after eighty hours of standing. Analysis ofthe system was made using a gas chromatographic instrument, modified bythe addition of a 10 cc. ion chamber in series with the conductivitycell and a Vibrating Reed Electrometer. The accompanying drawingillustrates the ,rate of tritiated methane formation in a systemuninhibited with NO. In the graph, the ordinate values at the left areratios of the areas under the tritiated methane peak to areas under thetritium peak divided by 10- No curve is plotted to illustrate theformation of tritiated methane in the system inhibited With NO asdescribed above, since, as has been said, none of the tritiated productwas detectible under instrumental conditions even after eighty hours ofstanding. In each case, the bulbs which were utilized had previouslybeen left in an annealing furnace for over forty-eight hours'at 500 C.and degassed for several hours by pumping in a high vacuum The selectionof NO as the scavenger to be used was, of course, in large part basedupon its reactivity with the free radicals formed in the mannerhereinbefore suggested. The NO, containing an odd number of electrons,combines instantly with neutral free radicals to produce normal nitrosocompounds. Moreover, the bimolecular gas reaction between free methyland nitric oxide proceeds easily at room temperature and has the lowactivation energy of nearly zero kilocalories. However, other scavengingcompounds such as ethylene, propylene, butene, iodine and hydrogeniodide can all be utilized eifectively. Since only the formation ofcompounds resulting from ,8 radiation is suppressed by the addition ofsuch free radical scavengers, labeling or tagging processes dependingupon the reaction of dissociation products of the (I-Ie T)+ ion with thecompound to be tagged, such as the Wilzbach labeling process, are notinterfered with by the scavenging compounds present in the tritium gas.

From the foregoing description, it will be perceived that the presentinvention provides a simple and eifective method of inhibiting thedetrimental effects of [3 radiation in systems containing radioactivematerials emitting such particles in the process of radioactive decay.Such systems can, therefore, be stored over substantially longer periodsof time without concern for the accelerated damaging of the radioactivematerial itself and the diluent which may be utilized in the system. Theunsought labeling of organic impurities which may be present is alsosubstantially eliminated.

As will be apparent to those skilled in the art upon reading theforegoing disclosure, many modifications, substitutions, and changes arepossible in practicing the process of the present invention. Some ofthese have been suggested in the description set forth above, and suchothers as may fall within the scope of the appended claims are deemed tobe within the spirit of the invention.

We claim:

1. The method of inhibiting molecular decomposition and undesirablechemical chain reactions in systems containing ,8 ray emittingradioactive elements, and a compound capable of forming free radicalsupon irradiation with beta photons, said compound being selected fromthe group consisting of hydrogen and normally gaseous parafiins whichcomprises introducing in said system a material selected from the groupconsisting of normally gaseous olefins, normally gaseous diolefins, NO,HI and I which will freely react with free radicals formed in saidsystem to produce stable compounds.

2. In a gaseous system including a compound containing at least onetritium atom, the method of inhibiting the formation of additionalcompounds as a result of radioactive decay of the tritium atom whichcomprises introducing to said system a radical-scavenging materialselected from the group consisting of normally gaseous olefins, normallygaseous diolefins, NO, HI and 1 which will readily react with freeradicals resulting from decomposition of a compound selected from thegroup consisting of hydrogen and normally gaseous parafiins and producedby said decay "to form stable compounds.

3. The method claimed in claim 2 wherein said radicalscavenging materialis selected to yield nongaseous compounds upon reaction with said freeradicals.

4. A method of inhibiting the formation of tritiated paraifinhydrocarbons in a gaseeous system containing tritium and parafiinhydrocarbons which comprises introducing to said system as aradical-scavenging material nitric oxide which Will react with freeradicals resulting from radioactive decay of tritium atoms to producestable compounds.

5. In a gaseous system containing tritium and traces of methane, themethod of inhibiting the formation of tritiated methane which comprisesadding to the system an amount of nitric oxide sufiicient to inhibit theformation of tritiated methane for the period of time desired.

6. In a gaseous system containing tritiated propane and propane, themethod of inhibiting decomposition of the tritiated propane and propane,and the forming of undesirable products of such decomposition whichcomprises introducing to the system a radical-scavenging materialselected from the group consisting of normally gaseous olefins, normallygaseous diolefins, NO, HI and I which will readily form stable compoundswith free radicals produced in the system as a result of radioactivedecay of tritium atoms.

7. In a gaseous system containing tritiated propane and propane themethod of inhibiting the formation of additional compounds duringstorage as a result of radioactive decay of tritium which comprisesintroducing into said system sufficient 1,3-butadiene to inhibit theformation of said additional compounds for the duration of the period ofstorage.

References Cited in the file of this patent Henglein: Nuclear ScienceAbstracts, vol. 14, Abstract No. 4335, March 15, 1960, which cites AECDocument NYC 2870, dated Dec. 4, 1959.

1. THE METHOD OF INHIBITING MOLECULAR DECOMPOSITION AND UNDESIRABLECHEMICAL CHAIN REACTIONS IN SYSTEMS CONTAINING B RAY EMITTING RDIOCATIVEELEMENTS, AND A COMPOUND CAPABLE OF FORMING FREE RADICALS UPONIRRADIATION WITH BETA PHOTONS, SAID COMPOUND BEING SELECTED FROM THEGROUP CONSISTING OF HYDROGEN AND NORMALLY GASEOUS PARAFFINS WHICHCOMPRISES INTRODUCING IN SAID SYSTEM A MATERIAL SELECTED FROM THE GROUPCONSISTING OF NORMALLY GASEOUS OLEFINS, NORAMLLY GASEOUS DIOLEFINS, NO,HI AND I2 WHICH WILL FREELY REACT WITH FREE RADICALS FORMED IN SAIDSYSTEM TO PRODUCE STABLE COMPOUNDS.